DESCRIPTION: The goal of this research is to understand the relationships among molecular structure, electron properties, and ground and excited state reactivity and relaxation dynamics of porphyrins and related ring systems. These relationships are central to the mechanisms of the diverse biological functions of tetrapyrrole chromophores, but are often difficult to investigate in detail in vivo. Therefore, research on the biological systems must be complemented by the study of tetrapyrroles in vitro. Time-resolved and steady state optical studies of model complexes are proposed to delineate functional properties of porphyrinic cofactors in biological systems such as the heme proteins and photosynthetic reaction center. Specific Aim 1 is to elucidate the electronic properties and excited state relaxation and electron transfer mechanisms of closely-spaced porphyrin dimers. Several tetrapyrrole dimers will be studied to probe key characteristics of biological complexes such as the bacteriochlorophyll special pair of the reaction center. The proposed studies will explore the effects of electronic asymmetry on the nature and relaxation kinetics of the lowest excited states, the role of the pi-overlap pattern in defining photophysical properties, and electron transfer involving strongly-coupled porphyrin dimers. Specific Aim 2 is to elucidate how nonplanar macrocycle distortions modulate the static and transient electronic properties of tetrapyrrole. Nonplanar distortions of the tetrapyrrole cofactors are widely observed in the protein complexes. Work from the PI s lab has shown that such distortions have dramatic effects on ground and excited state properties. Studies of electronic relaxation rates and pathways, photoinduced conformational excursions, and transient ligand binding/release and other properties will be extended to porphyrins with varying modes of nonplanar distotion (saddle, ruffle, dome, wave), chlorins (chlorophyll models), and iron porphyrins (heme models). Complementary ultrafast time resolved vibrational studies will give direct structural information. There have been few such studies for planar porphyrins and none for the nonplanar complexes. The combined studies will provide fundamental insights into the functional and spectroscopic properties of tetrapyrrole cofactors in vivo.